[BACK_INCOMPAT] `fabric.StaticCanvas#backgroundImage` and `fabric.StaticCanvas#overlayImage` are `fabric.Image` instances

[BACK_INCOMPAT] `fabric.StaticCanvas#backgroundImage` and `fabric.StaticCanvas#overlayImage` are `fabric.Image` instances. `fabric.StaticCanvas#backgroundImageOpacity`, `fabric.StaticCanvas#backgroundImageStretch`, `fabric.StaticCanvas#overlayImageLeft` and `fabric.StaticCanvas#overlayImageTop` were removed.

`backgroundImage` and `overlayImage` are now `fabric.Image` instances
New property `overlayColor` (analog to `backgroundColor`) - should be set with `canvas.setOverlayColor`
`backgroundImageOpacity` was removed => use `fabric.Image#opacity`
`overlayImageLeft ` was removed => use `fabric.Image#left`
`overlayImageTop ` was removed => use `fabric.Image#top`
`backgroundImageStretch` was removed => use `fabric.Image#width` and `fabric.Image#height`. If you scale your canvas you have to adjust the backgroundImage width/height manually.
Update SVG output.
Add 2nd parameter `firstLetterOnly` to `fabric.util.string.capitalize` => only first letter is transformed to uppercase (other letters stay untouched)
Add `preserveAspectRatio="none"` to `fabric.Image#toSVG` (otherwise streched backgroundImage and overlayImage didn't work as expected)
Update examples - TODO: Update jsfiddles for `setBackgroundImage` and `setOverlayImage`
Add unit test

Closes issue #270